- Awful acting, mainly Hayden's fault.
- Too much green screen.
- Two-dimensional characters.
- Gungans.
- Silly plot devices, even more silly than the original films.
- Taking a totally different direction than the original trilogy; way more action, less story and character development.
- Horrible dialogue.
And probably some more. I myself don't hate them as much as most people and I generally find them pretty entertaining. Heck, the one that's seen as the worst, Phantom Menace, is actually my favourite prequel film. The reasons are simple:
- Far less green screen than the other 2.
- Anakin is an annoying kid, yes, but way less annoying than Emo Anakin.
- Liam Neeson.
- Darth Maul.
- Podracing.
And I don't mind the midi-chlorians that much. By the Nine people, they're just indications of Force sensitivity. They don't decide whether you're an awesome Jedi or anything, it has barely if anything to do with the Jedi and I don't see how they could take away so much of their mysticism.
There's no reason to hate them that much, there are far worse things about those movies.